The frozen confection which is most often produced by an extrusion process is ice cream. It has a high sugar and high fat content and usually requires an emulsifier to be present in the composition. The mixture is typically subjected to high shear at a temperature of about −6° C. and is aerated to form a homogeneous dispersion of air and ice crystals. It is then typically chilled further to a temperature of around −19° C. and extruded or otherwise filled into a container.
Water ices are merely compositions comprising mainly water, typically also with sugar, colour, flavouring and stabilisers. Sorbets are aerated water ices. They contain no fat.
It is known to produce extruded ice confections which contain less fat than conventional ice-creams but unlike water ices/sorbets, they are not fat-free. This kind of product is typically prepared by making a mixture of the ingredients somewhat above freezing, for example at around 5° C. and then they are chilled to about −7° C. by a standard ice cream freezer followed by a further freezing step in a hardening tunnel to −18° C. or lower. It is also necessary for these products to be aerated prior to extrusion. Typically, they contain around 15% by weight of sugar and around 5% by weight of fat
A commercial ice making machine of the kind used to produce granular ice or small chips or blocks of ice typically consists of a chilled barrel through which runs a screw transporter, e.g. as disclosed in U.S. Pat. No. 3,678,702 or U.S. Pat. No. 6,134,908. Water is fed into one end of the barrel wherein it is frozen and the resultant ice transported to an exit die where the screw chips-off pieces of ice at a temperature of typically, around −1° C.
It is also known to extrude pure ice which has been pre-formed as a block, using a so-called ‘Instron” machine by forcing the block into a die of progressively narrowing internal diameter, whereby the resultant pressure melts the outside of the block but which then re-freezes to produce a continuous ice extrusion through the exit orifice.
U.S. Pat. No. 6,120,813 discloses “extrusion” of a fat free aerated water ice at temperatures around −5° C. from a scraped surface heat exchanger (a standard ice cream freezer). However, extrusion only refers to product exiting the heat exchanger under its own weight and/or under pressure of the incoming mixture which is to be frozen. The heat exchanger itself does not produce any force to extrude product. The resultant product is relatively soft.
US-A-2005/0214416 discloses extrusion of ice cream from a twin screw extruder, the product then being cut up and shaped into balls. It also describes that sorbets can be made in this way (when the extrusion temperature is preferably −16 to −20° C.). This kind of product is also relatively soft and has the consistency of an ice slush. US-A-2005/0214416 teaching is limited to an extruded material with a pasty consistency, while not giving any definition of what is meant by “pasty”. Secondly aeration at a level of 80% to 120% is needed and thirdly water ice composition is excluded. All these differences indicate that the extrusion process is only capable of processing soft/pasty material.
U.S. Pat. No. 6,228,412 discloses a mono-screw extrusion process for manufacturing frozen aerated products, particularly ice cream. The products have a total solids content of at least 30%, much of which is sugars. Therefore the products are quite soft and do not contain high levels of ice.
U.S. Pat. No. 6,207,213 discloses the preparation of frozen concentrated milk by means of an extrusion process. The concentrated milk must have a solids content of at least 20%. This document does not relate to frozen confections.
The applicants have sought to devise a process whereby water-based frozen products with low fat content and high ice content can be produced by an extrusion process without the need for aeration. This has now been achieved by the present invention. The products so formed can be extruded as a relatively hard, unitary and continuous extrudate.